1. Field of the Invention
The present invention relates generally to solar power systems for the conversion of sunlight into electrical energy, and, more particularly, to the use of III-V compound semiconductor solar cells.
2. Description of the Related Art
Commercially available silicon solar cells for terrestrial solar power application have efficiencies ranging from 8% to 15%. Compound semiconductor solar cells, based on III-V compounds, have 28% efficiency in normal operating conditions and 32.6% efficiency under concentration. Moreover, it is well known that concentrating solar energy onto the photovoltaic cell increases the cell's efficiency.
Terrestrial solar power systems currently use silicon solar cells in view of their low cost and widespread availability. Although compound semiconductor solar cells have been widely used in satellite applications, in which their power-to-weight efficiencies are more important than cost-per-watt considerations in selecting such devices, such solar cells have not yet been designed and configured for terrestrial systems, nor have terrestrial solar power systems been configured and optimized to utilize compound semiconductor solar cells.
In conventional solar cells constructed with silicon (Si) substrates, one electrical contact is typically placed on a light absorbing or front side of the solar cell and a second contact is placed on the back side of the cell. A photoactive semiconductor is disposed on a light-absorbing side of the substrate and includes one or more p-n junctions, which creates electron flow as light is absorbed within the cell.
The contact on the face of the cell where light enters is generally expanded in the form of a grid pattern over the surface of the front side and is generally composed of a good conductor such as a metal. The grid pattern does not cover the entire face of the cell since grid materials, though good electrical conductors, are generally not transparent to light.
The grid pattern on the face of the cell is generally widely spaced to allow light to enter the solar cell but not to the extent that the electrical contact layer will have difficulty collecting the current produced by the electron flow in the cell. The back electrical contact has not such diametrically opposing restrictions. The back contact simply functions as an electrical contact and thus typically covers the entire back surface of the cell. Because the back contact must be a very good electrical conductor, it is almost always made of metal layer.
The placement of both anode and cathode contacts on the back side of the cell simplifies the interconnection of individual solar cells in a horizontal array, in which the cells are electrically connected in series. Such back contact designs are known from PCT Patent Publication WO 2005/076960 A2 of Gee et al. for silicon cells, and U.S. patent application Ser. No. 11/109,016 filed Apr. 19, 2005, herein incorporated by reference, of the present assignee, for compound semiconductor solar cells.
Another aspect of terrestrial solar power system is the use of concentrators (such as lenses and mirrors) to focus the incoming sun rays onto the solar cell or solar cell array. The geometric design of such systems also requires a solar tracking mechanism, which allows the plane of the solar cell to continuously face the sun as the sun traverses the sky during the day, thereby optimizing the amount of sunlight impinging upon the cell.
Still another aspect of concentrator-based solar power cell configuration design is the design of heat dissipating structures or coolant techniques for dissipating the associated heat generated by the intense light impinging on the surface of the semiconductor body. Prior art designs, such as described in PCT International Publication No. WO 02/080286 A1, published Oct. 10, 2002, utilize a complex coolant flow path in thermal contact with the (silicon) photovoltaic cells.
Still another aspect of a solar cell system is the physical structure of the semiconductor material constituting the solar cell. Solar cells are often fabricated in vertical, multijunction structures, and disposed in horizontal arrays, with the individual solar cells connected together in an electrical series. The shape and structure of an array, as well as the number of cells it contains, are determined in part by the desired output voltage and current. One type of multijunction structure useful in the design according to the present invention is the inverted metamorphic solar cell structures, such as described in U.S. Pat. No. 6,951,819 (Iles et al.), M. W. Wanless et al, Lattice Mismatched Approaches for High Performance, III-V Photovoltaic Energy Converters (Conference Proceedings of the 31st IEEE Photovoltaic Specialists Conference, Jan. 3-7, 2005, IEEE Press, 2005); and U.S. Patent Application Publication No. 2007/0277873 A1 (Cornfeld et al.), and herein incorporated by reference